Method of forming a field effect transistor

ABSTRACT

A method of forming a field effect transistor includes forming a channel region within bulk semiconductive material of a semiconductor substrate. Source/drain regions are formed on opposing sides of the channel region. An insulative dielectric region is formed within the bulk semiconductive material proximately beneath at least one of the source/drain regions. A method of forming a field effect transistor includes providing a semiconductor-on-insulator substrate, said substrate comprising a layer of semiconductive material formed over a layer of insulative material. All of a portion of the semiconductive material layer and all of the insulative material layer directly beneath the portion are removed thereby creating a void in the semiconductive material layer and the insulative material layer. Semiconductive channel material is formed within the void. Opposing source/drain regions are provided laterally proximate the channel material. A gate is formed over the channel material. Integrated circuitry includes a bulk semiconductor substrate. A field effect transistor thereon includes a gate, a channel region in the bulk semiconductor substrate, and source/drain regions within the substrate on opposing sides of the channel region. A field isolation region is formed in the bulk semiconductor substrate and laterally adjoins with one of the source/drain regions. The field isolation region includes a portion which extends beneath at least some of the one source/drain region. Other aspects are contemplated.

TECHNICAL FIELD

This invention relates to methods of forming field effect transistors,to methods of forming integrated circuitry, and to integrated circuitry.

BACKGROUND OF THE INVENTION

Semiconductor processors continue to strive to reduce the size ofindividual electronic components, thereby enabling smaller and denserintegrated circuitry. One typical circuitry device is a field effecttransistor. Such typically includes opposing semiconductive source/drainregions of one conductivity type having a semiconductive channel regionof opposite conductivity type therebetween. A gate construction isreceived over the channel region. Current can be caused to flow betweenthe source/drain regions through the channel region by applying asuitable voltage to the gate.

The channel region is in some cases composed of background doped bulksemiconductive substrate or well material, which is also receivedimmediately beneath the opposite type doped source/drain regions. Thisresults in a parasitic capacitance developing between the bulksubstrate/well and the source/drain regions. This can adversely affectspeed and device operation, and becomes an increasingly adverse factoras device dimensions continue to decrease.

The invention was principally motivated in overcoming problemsassociated with the above-identified parasitic capacitance in bulk fieldeffect transistor devices. However, the invention is in no way solimited, nor limited to solving or reducing this or any other problemwhether identified/identifiable herein or elsewhere, with the inventiononly being limited by the accompanying claims as literally worded and asappropriately interpreted in accordance with the doctrine ofequivalents.

SUMMARY

This invention includes methods of forming field effect transistors,methods of forming integrated circuitry, and integrated circuitry. Inbut one implementation, a method of forming a field effect transistorincludes forming a channel region within bulk semiconductive material ofa semiconductor substrate. Source/drain regions are formed on opposingsides of the channel region. An insulative dielectric region is formedwithin the bulk semiconductive material proximately beneath at least oneof the source/drain regions.

In one implementation, a method of forming a field effect transistorincludes providing a semiconductor-on-insulator substrate, saidsubstrate comprising a layer of semiconductive material formed over alayer of insulative material. All of a portion of the semiconductivematerial layer and all of the insulative material layer directly beneaththe portion are removed thereby creating a void in the semiconductivematerial layer and the insulative material layer. Semiconductive channelmaterial is formed within the void. Opposing source/drain regions areprovided laterally proximate the channel material. A gate is formed overthe channel material.

In one implementation, integrated circuitry includes a bulksemiconductor substrate. A field effect transistor thereon includes agate, a channel region in the bulk semiconductor substrate, andsource/drain regions within the substrate on opposing sides of thechannel region. A field isolation region is formed in the bulksemiconductor substrate and laterally adjoins with one of thesource/drain regions. The field isolation region includes a portionwhich extends beneath at least some of the one source/drain region.

In one implementation, integrated circuitry includes a substrate havinga field effect transistor formed thereon. The transistor includes agate, a channel region, and source/drain regions on opposing sides ofthe channel region. First and second dielectric insulative materialmasses are received beneath and contact the source/drain regions. Thedielectric insulative material masses do not extend to beneath thechannel region.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic sectional view of a semiconductor waferfragment at one processing step in accordance with an aspect of theinvention.

FIG. 2 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 1.

FIG. 3 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 2.

FIG. 4 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 3.

FIG. 5 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 4.

FIG. 6 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 5.

FIG. 7 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 6.

FIG. 8 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 7.

FIG. 9 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 8.

FIG. 10 is a diagrammatic top view of the FIG. 9 wafer.

FIG. 11 is a diagrammatic sectional view of an alternate embodimentsemiconductor wafer fragment in accordance with an aspect of theinvention.

FIG. 12 is a diagrammatic sectional view of still another alternateembodiment semiconductor wafer fragment at a processing step inaccordance with an aspect of the invention.

FIG. 13 is a view of the FIG. 12 wafer at a processing step subsequentto that shown by FIG. 12.

FIG. 14 is a view of the FIG. 12 wafer at a processing step subsequentto that shown by FIG. 13.

FIG. 15 is a view of the FIG. 12 wafer at a processing step subsequentto that shown by FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

A method of forming integrated circuitry, including a field effecttransistor, is initially described in but only some aspects of theinvention in connection with FIGS. 1-10. Referring initially to FIG. 1,a semiconductor substrate is indicated generally with reference 10. Inthe context of this document, the term “semiconductor substrate” or“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove. Also in the context of this document unless otherwise indicated,the term “layer” includes the singular and the plural.

Substrate 10 comprises a bulk semiconductor substrate 12. An examplepreferred material is monocrystalline, such as monocrystalline siliconlightly doped with p-type material. In the context of this document, theterm “bulk” also includes doped well regions within such substrates.Bulk substrate 12 comprises a channel region 14 which is shown as beingmasked by a pad oxide layer 13 and a patterned block of masking material16. For an exemplary 0.15 micron transistor gate width, an exemplarythickness for layer 13 is 100 Angstroms. An exemplary preferred materialfor mask 16 is silicon nitride deposited to an exemplary thickness of900 Angstroms. An example width is 0.25 micron. In the illustrated andpreferred embodiment, masking material 16 extends laterally beyond thelateral confines of channel region 14. Such provides but one example offorming a channel region within bulk semiconductive material of asemiconductor substrate, and of masking the same.

Referring to FIG. 2, at least one trench is formed into the bulksemiconductor substrate on at least one side of the channel regionreceived within the bulk semiconductor substrate. Preferably and asshown, two trenches 18, 19 are formed into bulk semiconductor substrate12 on opposing sides of masked channel region 14. Such preferably occursby any existing or yet-to-be developed substantially anisotropic etchingtechnique. An exemplary preferred depth for the trench etching is 2500Angstroms.

Referring to FIG. 3, an insulative dielectric material 20 is depositedover masking material 16 and to within and overfilling trenches 18 and19. Exemplary and preferred processing includes sidewall oxidationeither before or after deposition of layer 20. An example and preferredmaterial for layer 20 is high-density plasma deposited oxide. Theinsulative dielectric material is preferably initially deposited tooverfill the trenches and then subsequently planarized at least tomasking material 16 to provide the preferred illustrated FIG. 3construction. Example planarizing techniques include chemical-mechanicalpolishing and etch back.

Referring to FIG. 4, portions of insulative dielectric material 20 areremoved from within trenches 18 and 19 effective to form at least one,and preferably two as shown, source/drain voids 22 and 24 on therespective sides of channel region 14. Such removal as shown is alsopreferably effective to expose bulk semiconductive material 12. Anexample preferred depth of voids 22 and 24 within insulative dielectricmaterial 20 is 1700 Angstroms. The preferred removal technique is atimed anisotropic etch, and with a photolithographic patterned maskbeing received over the non-etched portions of layer 20. In thepreferred embodiment, such effectively defines the outlines of thesource/drains of the transistor(s) being formed. Preferably and asshown, such removing forms an outer surface of insulative dielectricmaterial 20 to be planar at the base of such voids 22 and 24.

Referring to FIG. 5, source/drain semiconductive material 26 is formedwithin source/drain voids 22 and 24. Material 26 in but one embodimentcomprises monocrystalline material (by way of example only via epitaxialsilicon growth), and in but another embodiment comprises polycrystallinematerial, and in but another embodiment a mixture of monocrystalline andpolycrystalline. An exemplary preferred material is polycrystallinesilicon, preferably in situ conductively doped with an n-typeconductivity enhancing impurity during a chemical vapor deposition.Accordingly, in the preferred embodiment, source/drain material 26covers and physically contacts the previously exposed bulk semiconductorsubstrate material 12.

Referring to FIG. 6, deposited material 26 is planarized at least tomasking material 16. Example and preferred techniques includechemical-mechanical polishing and etch back.

Referring to FIG. 7, channel region 14 is unmasked preferably by etchingaway all of the masking material 16 and all of pad oxide layer 13.Further preferably as shown, some and only some of semiconductivematerial 26 is etched from the substrate. Such might occur in a singleor more etching step(s) depending on the chemistry utilized and thedesires of the processor, as readily determinable by the artisan. By wayof example only, an example etch chemistry which will etch polysiliconand silicon nitride in a substantially nonselective manner includesplasma CF₄, CH₂F₂ and He. In the subject example, the preferred amountof semiconductive material left is 900 Angstroms thick. Such providesbut one example of forming source/drain regions 30 and 32 on opposingsides of channel region 14. The upper surface of channel region 14 inFIG. 7 is preferably approximately 200 Angstroms beneath the uppersurfaces of regions 30 and 32, which are also preferably substantiallyplanar.

Referring to FIG. 8, a gate 34 is formed over channel region 14.Preferably as shown, a gate dielectric layer 36, for example silicondioxide, is first formed over channel region 14. A gate stack is thenformed thereover, preferably comprising a conductively doped polysiliconlayer 38 and a conductive silicide layer 40 (for example WSi_(x)) and anitride capping layer 42. Thereafter, at least one pocket implanting isconducted to provide at least one pocket implant region intermediatesource/drain semiconductive material 26 and channel region 14. In theillustrated and preferred example, exemplary pocket implants includesource/drain extension (SDE) implant regions 44 having a thickness of500 Angstroms, and halo implant regions 46 provided therebeneath havingan approximate thickness of 500 Angstroms and to extend belowsource/drain regions 30, 32. Insulative spacers are subsequently addedas shown. Rapid thermal processing is preferably conducted at somepoint, as is conventional.

Referring to FIGS. 9 and 10, subsequent exemplary processing isillustrated. Depicted is the provision and planarizing of an insulativedielectric layer 48, for example borophosphosilicate glass (BPSG).Contact openings have been formed therethrough and plugged withconductive material to form source/drain contacts 50.

The above-described embodiment provides but one example of providing aninsulative dielectric region within bulk semiconductive material 12proximately beneath at least one of the source/drain regions. Preferablyand as shown, such insulative dielectric region is formed beneath bothsource/drain regions and physically contacts the subject source/drainregions. Further in the described and preferred embodiment, forming ofthe insulative dielectric region beneath the source/drain regions occursprior to forming the source/drain regions, and includes at least somedepositing of an insulative dielectric layer. Such preferred processingalso depicts the formation of gate 34 after forming the source/drainsemiconductive material.

Further, the illustrated construction provides but one example of novelintegrated circuitry independent of the method of fabrication. Suchcomprises a bulk semiconductor substrate including a field effecttransistor comprising a gate, a channel region in the bulk semiconductorsubstrate, and source/drain regions within the substrate on opposingsides of the channel region. At least one field isolation region isformed in the bulk semiconductor substrate and laterally adjoins withone of the source/drain regions. The field isolation region includessome portion 54 which extends beneath at least some of the source/drainregion (FIG. 9). In the illustrated and preferred embodiment, the fieldisolation region portions 54 contact the source/drain regionstherebeneath. Further preferably, field isolation region portion 54extends beneath at least a majority of the one source/drain region andeven more preferably extends beneath at least 90% of the source/drainregions. The illustrated example, shows greater than 95% coverage byportions 54 beneath the source/drain regions. Further preferably and asshown, each field isolation region portion 54 extends beneath less thanall of the source/drain region. Further, at least one pocket implantregion is received intermediate the source/drain region and the channelregion.

FIG. 11 illustrates but one exemplary alternate embodiment whichincludes forming the insulative dielectric region after forming thesource/drain regions. Like numerals from the first-described embodimentare utilized where appropriate, with differences being indicated by thesuffix “a” or with different numerals. Substrate 10 a comprisessource/drain regions 30 a and 32 a formed within, a bulk monocrystallinesilicon substrate 12 a. An implant masking construction 58 is formedover gate 34. Substrate 10 a is then subjected to a suitable ionimplantation whereby material is ion implanted into bulk semiconductivesubstrate material 12 a which is either insulative dielectric materialor a material which reacts with the bulk semiconductor material to forman insulative dielectric material. FIG. 11 depicts regions 59 formedthereby. Such processing might occur either before or after formingsource/drain regions 30 a and 32 a. An example implant would be ofoxygen atoms, for example at a dose of 4×10¹⁷ atoms or ions per cubiccentimeter at a suitable energy to achieve desired depth, and preferablyfollowed by an anneal.

Yet but one additional alternate embodiment of forming integratedcircuitry, including the forming of a field effect transistor, isdescribed with reference to FIGS. 12-15. FIG. 12 illustrates asemiconductor-on-insulator substrate 60. Such comprises, in thepreferred example, a bulk monocrystalline silicon substrate wafer 61having a layer 62 of insulative material formed thereover. An examplematerial is silicon dioxide. A layer 64 of semiconductive material isformed over layer 62. An example preferred material for layer 64 issilicon, preferably elemental silicon, such as monocrystalline orpolycrystalline silicon.

Referring to FIG. 13, all of a portion of semiconductive material layer64 and all of insulative material layer 62 immediately therebeneath areremoved, thereby creating a void 65 in semiconductive material layer 62.Such removing preferably occurs by photolithographic patterning of amasking layer and subsequent conventional or yet-to-be developed etchingthereof. Such removing preferably exposes bulk monocrystalline silicon61 of substrate 60 as shown.

Referring to FIG. 14, semiconductive channel material 66 is formedwithin void 65. The example and preferred technique, where substratematerial 61 comprises monocrystalline silicon, is conventional oryet-to-be-developed epitaxial silicon growth within void 65 from bulkmonocrystalline silicon 61. Deposited polysilicon is but one alternateexample.

Referring to FIG. 15, a gate dielectric layer 68 and a gate construction70 are provided over channel material 66. Opposing source/drain regions72 and 74 are provided laterally proximate channel material 66. Asshown, such preferably constitute a portion of semiconductive materiallayer 64. Such might be formed by ion implantation or other doping withor without masking. Further, such doping might occur prior to forminggate 70, prior to forming semiconductive channel material 66 or prior toforming void 65. One example preferred process would be to dopesemiconductor material layer 64 to a desired source/drain concentrationprior to forming void 65, whereby the removing to form such void removessemiconductive material previously subjected to such doping.

Such provides but one exemplary alternate method embodiment, and as welldepicts integrated circuitry construction in accordance with aspects ofthe invention independent of the method of fabrication. Such integratedcircuitry comprises a substrate having a field effect transistor formedthereon. FIG. 15 illustrates dielectric insulative material masses 75and 76 received beneath and contacting source/drain regions 72 and 74,with such dielectric insulative material masses not extending to beneathchannel region 66. FIG. 9 also illustrates but one additional exemplaryembodiment comprising first and second dielectric insulative materialmasses which are received beneath and contact source/drain regions, withsuch masses not extending to beneath the channel region.

By way of example only, one or more of the above embodiments may achieveone or more benefits. However, no one or combination of these benefitsconstitutes a requirement or subject matter of the accompanying claims.A first exemplary benefit includes reduction or essential elimination ofjunction capacitance beneath the source/drain regions, particularly inbulk semiconductor processing. Junction leakage is also reduced oreffectively eliminated, preferably. Further with respect to bulkprocessing, junction capacitance can be significantly reduced comparedto semiconductor-on-insulator processing. Further, the above-describedprocessing can result in a reduction of short channel effects comparedto other bulk semiconductor field effect transistor processing.

Another hopefully achieved advantage is improvement in active areaisolation. Such can effectively occur by an essential lateral extensionof the isolation region into what previously was a total bulk activearea beneath the source/drain regions in bulk wafer processing.

Further, the above processing and structure can provide for reduction orelimination of floating body effects, which still can occur in fullydepleted semiconductor-on-insulator structures. Further, the aboveprocessing can be used to fully integrate with borderless/low leakagecontacts where the risk of over etch into underlying substrate can beeffectively eliminated by the provision of the dielectric regionimmediately and contacting the source/drain junctions.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A method of forming a field effect transistorcomprising: forming at least one trench into a bulk semiconductorsubstrate on at least one side of a channel region received within thebulk semiconductor substrate; depositing an insulative dielectricmaterial to within the at least one trench; removing a portion of theinsulative dielectric material within the at least one trench effectiveto form at least one source/drain void within the insulative dielectricmaterial on the at least one side of the channel region, the removingforming the one source/drain void to comprise an exposed sidewallsurface of bulk semiconductor substrate material; forming source/drainsemiconductive material within the at least one source/drain void and inphysical contact with the exposed sidewall surface of the bulksemiconductor substrate material; and forming a gate over the channelregion.
 2. The method of claim 1 wherein the insulative dielectricmaterial comprises silicon dioxide.
 3. The method of claim 1 comprisingforming the gate after forming the source/drain semiconductive material.4. The method of claim 1 wherein the removing forms an outer surface ofthe insulative dielectric material to be substantially planar within thevoid.
 5. A method of forming a field effect transistor comprising:forming trenches into a bulk semiconductor substrate on opposing sidesof a channel region received within the bulk semiconductor substrate;depositing an insulative dielectric material to within the trenches;removing portions of the insulative dielectric material within thetrenches effective to form source/drain voids within the insulativedielectric material on opposing sides of the channel region, theremoving forming the source/drain voids to comprise exposed sidewallsurfaces of bulk semiconductor substrate material on opposing sides ofthe channel region; forming source/drain semiconductive material withinthe source/drain voids and in physical contact with the exposed sidewallsurfaces of the bulk semiconductor substrate material on opposing sidesof the channel region; and forming a gate over the channel region. 6.The method of claim 5 further comprising forming at least one pocketimplant region intermediate the source/drain semiconductive material andthe channel region.
 7. The method of claim 5 further comprising formingat least one pocket implant region in bulk semiconductor materialintermediate the source/drain semiconductive material and the channelregion.
 8. The method of claim 5 comprising forming the gate afterforming the source/drain semiconductive material.
 9. The method of claim5 wherein the removing forms an outer surface of the insulativedielectric material to be substantially planar within the void.
 10. Amethod of forming a field effect transistor sequentially comprising:masking a channel region in a bulk semiconductor substrate; formingtrenches into the bulk semiconductor substrate on opposing sides of themasked channel region; depositing an insulative dielectric material towithin the trenches; removing portions of the insulative dielectricmaterial within the trenches effective to form source/drain voids withinthe insulative dielectric material on opposing sides of the maskedchannel region, the removing forming the source/drain voids to compriseexposed sidewall surfaces of bulk semiconductor substrate material onopposing sides of the channel region; forming source/drainsemiconductive material within the source/drain voids and in physicalcontact with the exposed sidewall surfaces of the bulk semiconductorsubstrate material on opposing sides of the channel region; andunmasking the channel region and forming a gate thereover.
 11. Themethod of claim 10 comprising forming the source/drain regions to bepolycrystalline and the channel region to be monocrystalline.
 12. Themethod of claim 10 comprising forming the source/drain regions to bemonocrystalline the channel region to be monocrystalline.
 13. The methodof claim 10 comprising forming both the source/drain regions and thechannel regions to comprise crystalline elemental silicon.
 14. Themethod of claim 10 wherein the unmasking occurs after forming thesource/drain conductive material.
 15. A method of forming integratedcircuitry sequentially comprising: masking a channel region in a bulksemiconductor substrate with a masking material; etching trenches intothe bulk semiconductor substrate on opposing sides of the masked channelregion using the masking material as a mask; depositing an insulativedielectric material over the masking material and to within andoverfilling the trenches; planarizing the deposited insulativedielectric material at least to the masking material; etching theinsulative dielectric material within the trenches effective to formsource/drain voids within the insulative dielectric material on opposingsides of the masked channel region, the etching forming the source/drainvoids to comprise exposed sidewall surfaces of bulk semiconductorsubstrate material on opposing sides of the channel region; depositingsemiconductive material over the masking material and to within andoverfilling the source/drain voids, the deposited semiconductivematerial received with the source/drain voids being in physical contactwith the exposed sidewall surfaces of the bulk semiconductor substratematerial on opposing sides of the channel region; planarizing thedeposited semiconductive material at least to the masking material;etching all of the masking material and only some of the semiconductivematerial from the substrate; forming a gate dielectric layer over thechannel region; and forming a transistor gate over the gate dielectriclayer over the channel region.
 16. The method of claim 15 furthercomprising forming at least one pocket implant region in bulksemiconductor material intermediate the deposited semiconductivematerial and the channel region.
 17. The method of claim 15 wherein bothplanarizings comprise chemical-mechanical polishing.
 18. The method ofclaim 15 wherein the semiconductive material is polycrystalline.
 19. Themethod of claim 15 wherein the semiconductive material comprisespolycrystalline silicon.
 20. The method of claim 15 wherein thesemiconductive material is comprises polycrystalline silicon which is insitu during depositing.
 21. The method of claim 15 wherein the etchingof only some of the semiconductive material forms an outer surfacethereof to be substantially planar.